What's happened to my cat's leg? :(

I'm taking my cat to the vets later as I have noticed today that she is slower than normal (she's only 4) normally she's jumping around all over. She's been quite lethargic and I put it down to the weather. But she doesn't seem to be jumping like normal. And when she walks she's kind of stalling. Not limping and I cant work out if its her left or right back leg. So her hips kind of drop? Not explaining very well.

She is a house cat along with her sister but they've been going in the garden these last few days, but generally just sunbathing. they are fair weather cats, havent even been near the door at all today due to the rain! but when the sun's out they are calling me to let them out, so nothing I can think of in terms of injuring herself.

She's eating normally, and just heard her get into her radiator bed. She's just not herself and I'm worried she's in pain I'm not normally home in the day.

Thank you. I did try to have a look earlier at her pads but couldn't see anything.

She has flummoxed the vet. She's had a good check over and the vet agrees that she definitely is walking 'right' but she couldn't find anything untoward. She's had an anti-inflammatory injection that should last 24 hours and we've booked a follow up for next week. The vet did mention x-rays but said she'd be reluctant to do one immediately as I've only noticed this within the last day or so. She said it could be a sprain but its just wait and see. But she said at present she wasn't overly concerned